[backport v2.9.3] Side Menu Cluster List performance badly at scale

[github-actions]

This is a backport issue for #11993, automatically created via GitHub Actions workflow initiated by @richard-cox

Original issue body:

Setup

• Rancher version: 2.9.1 (and before)

Describe the bug

• When there are lots of clusters (~700) logging in takes a number of seconds
• Some of this is generic processing of a large number of resources (objectify, cache, etc), however some specific to the top level menu
  • Once the resources are fetched we spend ~3 seconds processing resources
  • 
• Of the 3 seconds process we spend over 1 second in a top level menu computed property clusters
  • 
• In clusters we iterate over all clusters a number of times. We also convert them to an object ready to show on screen.
• This will in theory go away with vai (we shouldn't ever fetch that many resources). Until then...
• Suggestion 1
  • recreating the issue
  • only map the objects we're going to show on screen, this could be a very small number
  • attempt to reproduce with fix
• Suggestion 2
  • less loops!

Note - clusters runs often, perhaps on cluster state change? each time it consumes over second of cpu

To Reproduce

• Load ~700 clusters (they could be RKE2 Custom)
• refresh on login page
• log in

Result

• should in theory be a large time spent in clusters

Expected Result

• almost no time in `clusters

Additional context

• If vai changes have merged this file will have already been reworked

[gaktive]

@richard-cox should we still try to get this in for this release if the 2.10.0 work is still in draft? or should we look at the next one: 2.9.4?

[gaktive]

Bot is moving tickets around unexpectedly. Need to sweep through for patterns and file a tech debt ticket to investigate.

[richard-cox]

Release Note (combined with #11999)

The performance of the Clusters lists in the Home page and the Side Menu has greatly improved when there hundreds of clusters.

[IsaSih]

Tests pass on
v2.9-99b2583e3370321a922e29a11ac5ff9f845baeb6-head